Furfural and related compounds are useful precursors and starting materials for industrial chemicals for use as pharmaceuticals, herbicides, stabilizers, and polymers. The current furfural manufacturing process utilizes biomass such as corn cob, sugar cane bagasse, switchgrass or wood waste as a raw material feedstock for obtaining glucose, glucose oligomers, cellulose, xylose, xylose oligomers, arabinose, hemicellulose, and other C5 and C6 sugar monomers, dimers, oligomers, and polymers.
The hemicellulose and cellulose are hydrolyzed under acidic conditions to their constituent sugars, such as glucose, xylose, mannose, galactose, rhamnose, and arabinose. Xylose, which is a pentose (i.e., a C5 monosaccharide) is the sugar present in the largest amount in hemicellulose. In a similar aqueous acidic environment, the C5 sugars are subsequently dehydrated and cyclized to furfural. Under similar conditions, C6 sugars can be hydrolyzed and converted in low yields to furfural.
In a process disclosed by John W. Dunning et al. in U.S. Pat. No. 2,559,607, aqueous pentose (1.5-10%) was converted to furfural under pressure using sulfuric acid (1.5-5 wt %) and temperatures from 140° C. to 165° C. Three different methods were used to separate the furfural product from the pentose solution. In the first two, furfural was removed by extraction into toluene; in the third method, the furfural was removed by steam stripping. Dunning et al. claimed yields as high as 80% using the first two methods and slightly lower yields using the third method. These yields were based on the amount of xylose converted in the process, which typically required reprocessing the soluble xylose stream several times to get up to only about 50% xylose conversion.
In a process disclosed by Andrew P. Dunlop (U.S. Pat. No. 2,536,732), furfural was produced in yields of up to 82% where aqueous xylose solutions were fed to solvents substantially insoluble in water (restricted to the class of solvents alkylated benzenes, polyhalogenated benzenes, and chlorinated biphenyls), wherein the reaction pot was a biphasic reaction mixture of aqueous xylose and solvent as separate phases. Yields of 82% were obtained at 0.39 parts of xylose per 100 parts of water solution, but only 49% yield was obtained at higher concentrations of xylose feed (9.38 parts of xylose per 100 parts of aqueous solution).
In a process disclosed by David J. Medeiros et al. in U.S. Pat. No. 4,533,743, aqueous pentose solution was reacted at high temperature and pressure in the presence of a mineral acid catalyst to maximize furfural yield and selectivity. The process utilized a plug flow reactor and a combination of four conditions: The concentration of pentose in the pentose-aqueous feed solution before entry into the reactor was between 1 and 10 percent by weight of the aqueous solution before the addition of acid; the concentration of the mineral acid in the reactor was between 0.05 and 0.2 normality before entry into the reactor; the reactor was operated at a temperature between 220° C. and 300° C.; and the residence time of the pentose in the reactor was between 0.5 and 100 seconds. The reactor pressure was high enough to prevent vaporization of the aqueous solutions at the high temperatures used, between about 1000 and 2000 psi (6.895 and 13.79 MPa). In one configuration of the process, furfural produced from a xylose stream was separated with a water immiscible extraction solvent. The yield of furfural was 66% after one pass. Because incomplete conversion of the xylose took place, the aqueous solution could be recycled for additional yield. The selectivity of the xylose converted was 73%. Faster flow rates increased selectivity at the expense of lowered conversion, suggesting that maximum yields using multiple cycles would reach 80-85%. In a second configuration, the aqueous xylose solution was mixed with an immiscible solvent, toluene, before reaction. The conversion was 98% and the yield was 71%.
In a process disclosed by Takeshi Suzuki, et al. (Applied Catalysis A: General, 2011, Vol. 408, pp 117-124), solid acid catalysts were employed to convert xylose to furfural; solid acid catalyst processes described in the art require high reaction temperature and pressure and/or supercritical solvent to attain the selective production of furfural with a high yield, and are often deactivated by collection of humins on the catalyst.
In a process disclosed by Haruo Kawamoto, et al. (J. Wood Science, 2007, Vol. 53, pp 127-133) pyrolysis of the C6 sugar oligosaccharide cellulose in sulfolane with an acid catalyst at 200° C. and with steam yielded furfural in ca. 27% yield; without steam or water added yields of furfural were ≦20%.
There remains a need for a process to produce furfural at both high yield and high conversion, capable of operation in a batch or continuous mode, and which allows for removal of soluble humin byproduct in a batch or continuous mode. It is also desirable that such a process be carried out without the need for high pressure equipment.